I wish they just made Brock’s parents dead because God this is so messed up.
Judging by how old Brock’s youngest siblings are, his parents abandoned them when they were really young. Brock was maybe a preteen then? So soon after he got his Onix (a gift from Flint, his dad, on his tenth birthday), he was left to raise a gaggle of siblings by himself and to run the Pewter Gym.
He probably didn’t get to hang out around kids his age, which is probably the reason why he has no idea how to deal with his attraction to women. He didn’t have any male role model to teach him how to show his attraction in a non-creepy/annoying way, and he wasn’t able to learn from his peers either, because he was too busy raising his siblings and being the Gym Leader. After that, his closest group of friends (aka Ash and co) were all younger than him and the best they could do is pull on his ear when he got creepy/annoying again.
Later on when his parents do come back they’re still irresponsible and act immaturely, even compared to Brock’s younger siblings. Lola redecorates and re-types the gym on her whims, Flint just gives into her demands, and it’s up to Brock and his siblings to save the gym from utter destruction. They take Brock’s Pokémon on their vacation and leave their kids behind without even making sure someone’s there to take care of them (it was sheer coincidence Brock and the gang came to Pewter at the time). It’s Forrest, not Flint, who defends the gym when a Nurse Joy comes around with a fucking Legendary as a test for the gym’s competence (even if Forrest was the Gym Leader at the time, he should’ve stepped in as he realized his preteen son was facing a Latias!)
TLDR Flint and Lola are terrible parents and I truly wonder how they could produce someone as wonderful and responsible as Brock is.